1. Field of the Invention
This invention relates to a two-cycle internal combustion engine of the type which comprises a plurality of scavenging ports repetitively opened and closed by piston reciprocation for forcibly charging the cylinder while also displacing (scavenging) the residual gas.
2. Description of the Related Art
As is well known, a two-cycle internal combustion engine completes one cycle by two strokes (i.e., one reciprocation) of a piston within a cylinder. More specifically, combustion or explosion takes place at the upper deadpoint of the piston stroke followed by a downward stroke of the piston. As the piston approaches the lower deadpoint, an exhaust port is opened for discharging the combustion gas, which is followed by opening of scavenging ports for introducing a new charge which displaces or scavenges a residual portion of the combustion gas. Then, the piston makes an upward stroke for compressing the new charge, and the compressed new charge is again ignited at the upper deadpoint of the piston.
A typical prior art two-cycle engine is disclosed in FIGS. 8 through 12 of JP-A-1-102318 (Laid-open: Apr. 13, 1990) for example. Specifically, the two-cycle engine disclosed in this Japanese document comprises a cylinder receiving a reciprocating piston and having a top portion provided with a combustion chamber in which a spark plug faces, an exhaust port laterally opening into the cylinder, and an opposite pair of scavenging ports opening into the cylinder at both sides of the exhaust port.
With the prior art two-cycle engine described above, each of the scavenging ports is oriented to direct scavenging gas away from the exhaust port. Thus, the scavenging gas moves along a standing loop which first extends away from the exhaust port, then approaches the spark plug, and finally reaches the exhaust port. However, since the scavenging gas flows simply along the standing loop without any whirling or twisting, it has been found that a portion of the scavenging gas (i.e., charge gas) tends to blow by immediately into the exhaust port, thereby resulting in a decrease of the trapping efficiency (defined as the ratio of a cylinder-trapped portion of a new charge relative to the new charge after scavenging). Further, due to the formation of the standing loop, a portion of the combustion gas within the loop tends to be left non-scavenged, thereby leading to a decrease of the scavenging efficiency (defined as the ratio of a cylinder-trapped portion of a new charge relative to the total gas amount remaining in the cylinder after scavenging) accompanied by poor combustion.
JP-A-4-5429 (Laid-open: Jan. 9, 1992) discloses another prior art two-cycle engine which comprises a cylinder receiving a reciprocating piston and having a top portion provided with a combustion chamber in which a spark plug faces, an exhaust port laterally opening into the cylinder, a first pair of scavenging ports opening into the cylinder at one side of the exhaust port, a second pair of scavenging ports opening into the cylinder at another side of the exhaust port opposite to the first pair of scavenging ports, and a booster port opening into the cylinder at a position diametrically opposite to the exahust port. Again, each of the scavenging ports is oriented to direct scavenging gas away from the exhaust port, so that the scavenging gas basically moves along a standing loop which first extends away from the exhaust port, then approaches the spark plug, and finally reaches the exhaust port.
On the other hand, the booster port is oriented to direct a portion of the scavenging gas obliquely upward toward the spark plug. Thus, the scavenging gas portion introduced through the booster port stabilizes and accelerates the movement of the scavenging gas as a whole along the standing loop. Such stabilization and acceleration of the loop flow have been found effective for improving the trapping and scavenging efficiency (hence combustion performance) of the engine when the engine operates at a low delivery ratio (under a low load condition for example). Conversely, if the engine operates at a high delivery ratio (under a high load condition for example), the booster port excessively accelerates the loop scavenging flow, consequently leading to a decrease of the trapping and scavenging efficiency (hence poor combustion).